So when God wanted to make the unchanging nature of His purpose very clear to the heirs of the promise, He guaranteed it with an oath.
17. God, willing, etc. See how kindly God as a gracious Father accommodates himself to our slowness to believe; as he sees that we rest not on his simple word, that he might more fully impress it on our hearts he adds an oath.
The apostle, having applied himself to the fears of the Hebrews, in order to excite their diligence and prevent their apostasy, now proceeds to apply himself to their hopes, and candidly declares the good hope he had concerning them, that they would persevere; and proposes to them the great encouragements they had in the way of their duty. I.
Commenting on Hebrews 6:9-20
That by two immutable things,.... God's counsel and oath, which never change and alter, and from which he never varies: in which it was impossible for God to lie; fail or deceive, nor, indeed, in anything else, besides his counsels, promises, and oath, see Tit 1:2 we might have a strong consolation; the saints often stand in need of consolation, by reason of sin, Satan...